1. Field of the Invention
The present invention pertains to circuits for translating voltage signal levels; for example, translating RS232 voltage signal levels to Transistor-Transistor Logic (TTL) voltage signal levels.
2. Description of Prior Art
With the rapid development of communication technologies, the Electronic Industries Association (EIA) and the Telecommunication Industry Association (TIA) have developed a number of standards to simplify data communication. The RS232 standard is one of the most widely known communication standards. Voltage levels of voltage signals that conform to the RS232 standard range from −12V to +12V. Nowadays digital technology is widely used, and this needs to implement logic signal families that have voltage levels different from that of the RS232 voltage signals. Most logic signal families utilize narrower and single-sign voltage levels. Voltage signals in these logic signal families are compatible with the operation requirements of Metal Oxide Semiconductor (MOS) transistors, and exhibit reduced noise levels typically encountered in existing digital technologies. One of such logic signal families is the TTL signal family, which recognizes voltage signals ranging from 0 to +5V.
Because the voltage range of RS232 voltage signals is different from that of TTL voltage signals, the RS232 voltage signals and the TTL voltage signals are incompatible. Therefore, devices utilizing the RS232 standard (RS232 devices) cannot communicate with devices utilizing the TTL signal family (TTL devices), at least not directly.
Although communication technology is gradually moving away from the RS232 standard, this standard is still employed in a wide variety of applications. Intermediate devices for permitting the RS232 devices to communicate with the TTL devices by translating voltage signal levels are needed.
Conventional devices for translating voltage signal levels are level translation ICs. These ICs translate voltage signal levels between RS232 devices and TTL devices in order to permit the RS232 devices to communicate with the TTL devices. Thus, when TTL voltage signals are transmitted from a TTL device to the level translation IC, the level translation IC converts the TTL voltage signals to RS232 voltage signals, and sends the RS232 voltage signals to the RS232 device. Conversely, when RS232 voltage signals are transmitted from the RS232 device to the level translation IC, the level translation IC converts the RS232 voltage signals to TTL voltage signals, and sends the TTL voltage signals to the TTL device. The level translation IC commonly employed for this purpose needs to be connected to both a +5V power supply and a +12V power supply.
Although level translation ICs are suitable for some applications to perform bidirectional voltage level translations between TTL voltage signals and RS232 voltage signals, they have a number of disadvantages. First, level translation ICs are relatively expensive and complex, and are particularly cost-inefficient in systems where they only perform voltage level translation between RS232 voltage signals and TTL voltage signals. Second, a conventional level translation IC must be connected to the power supplies of both the RS232 device and the TTL device in order to perform voltage level translation.
Therefore, there is a need for a simple and inexpensive circuit to translate voltage signal levels and permit communication among devices utilizing different logic signal families.